#40909 added `CREATE_BREAKAWAY_FROM_JOB` to `windows_detach_flags()`,
which fixed the headline bug (gateway dies after Desktop GUI update
and never comes back). The flag's own docstring acknowledges that
restrictive parent job objects can still refuse breakaway with
`ERROR_ACCESS_DENIED`, surfacing as `OSError` on the `subprocess.Popen`
call:
"Callers in this codebase already wrap detached spawns in
try/except OSError and fall back to a cmd.exe wrapper, so the
breakaway-denied case degrades gracefully rather than crashing."
That's true for `_spawn_detached` in `gateway_windows.py` (the
`hermes gateway start` path), which has both the breakaway bit AND a
retry-without-breakaway fallback. It's NOT true for the post-update
watcher path in `launch_detached_profile_gateway_restart`
(`hermes_cli/gateway.py`), which only has `except OSError: return
False` and gives up entirely. If a user's shell/terminal/container
wraps Hermes in a breakaway-denying job, the gateway-respawn watcher
silently fails to launch instead of trying again without breakaway.
This PR closes that gap and adds the regression tests that were
missing from the original fix.
## Changes
### `hermes_cli/_subprocess_compat.py`
Adds a sibling helper `windows_detach_flags_without_breakaway()` so
callers can express the fallback symbolically (via the helper) rather
than coding the magic `& ~0x01000000` mask at every site. Documented
on `windows_detach_flags` and `windows_detach_flags_without_breakaway`
with the recommended try/except pattern.
### `hermes_cli/gateway.py::launch_detached_profile_gateway_restart`
Two changes, both aligned with the canonical pattern in
`gateway_windows._spawn_detached`:
1. The outer watcher Popen now wraps in `try/except OSError`, and on
failure retries with `windows_detach_flags_without_breakaway()`
(POSIX never reaches this branch — `start_new_session=True` can't
raise OSError).
2. The inlined respawn payload (the `python -c` watcher) also
wraps its CreateProcess in try/except OSError and retries with
`_flags & ~_CREATE_BREAKAWAY_FROM_JOB` on failure. This matters
because the watcher's job-object inheritance is independent of the
outer process's — even if the outer Popen succeeds with breakaway,
the respawned gateway might inherit a job that doesn't.
### Regression tests in `tests/tools/test_windows_native_support.py`
#40909 shipped the fix without any test that the breakaway bit is
present (the existing `test_windows_detach_flags_has_expected_win32_bits`
asserted only the three legacy bits). Four new tests close that:
- `test_windows_detach_flags_includes_breakaway_from_job` — explicit
assertion that the breakaway bit is in the default bundle, with the
rationale spelled out in the docstring so a future maintainer
staring at this test understands why removing it would resurrect
the gateway-dies-after-GUI-update bug.
- `test_windows_detach_flags_without_breakaway_drops_only_that_bit`
— fallback payload keeps the other three detach bits intact.
- `test_launch_detached_profile_gateway_restart_inlined_watcher_uses_breakaway`
— static-text check on the stringified watcher payload. The inlined
Python program isn't reachable via normal import-time inspection
because it lives in a `textwrap.dedent("""...""")` literal that
gets passed to a separate `python -c` interpreter. Asserting that
both `_CREATE_BREAKAWAY_FROM_JOB` (symbolic) and `0x01000000` (hex
literal) appear inside the dedent block is a sufficient regression
guard against accidental refactors.
- `test_launch_detached_profile_gateway_restart_outer_popen_has_access_denied_fallback`
— static check that this PR's fallback retry is wired up
symbolically. Without standing up a real Windows job object that
refuses breakaway, we can't trigger the OSError in a unit test;
the text guard catches the case where a future refactor removes
the helper import or the `& ~_CREATE_BREAKAWAY_FROM_JOB` retry.
Also extends `test_windows_detach_flags_has_expected_win32_bits` to
include the breakaway bit assertion and updates
`test_windows_flags_zero_on_posix` to cover the new helper.
## Tests
Locally on Windows: 8/8 in the `-k "detach or breakaway or
popen_kwargs or launch_detached or gateway_run_update or
hermes_cli_gateway"` slice pass.
Broader `tests/hermes_cli/test_gateway*.py + test_windows_native_support.py`:
172 passed, 10 failed. All 10 failures are pre-existing POSIX-only
tests running on a Windows host (os.geteuid, SIGKILL fallback,
is_linux fixture mismatches). Stashing this PR and re-running on bare
post-#40909 main reproduces all 10 identically — none are regressions.
POSIX paths unchanged: `windows_detach_flags()` and
`windows_detach_flags_without_breakaway()` both return 0 off Windows,
`windows_detach_popen_kwargs()` still yields `{"start_new_session": True}`.
## Out of scope
- The other detached-spawn site in `hermes_cli/gateway.py` (around
line 3068) also uses `windows_detach_popen_kwargs()` + `except
OSError`. It deserves the same fallback treatment but the codepath
is different enough (not the update-flow watcher) that it warrants
a separate PR with its own scrutiny.
- `gateway/run.py` has Windows branches with `windows_detach_popen_kwargs`
too — same reasoning.
## Context
Follow-up to #40909 (merged). I had a parallel PR (#40934, closed)
that duplicated the core breakaway fix; the bits unique to that PR
that #40909 didn't cover are the contents of this one. Closing #40934
and opening this slimmed-down version as the focused follow-up.
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